Over the past decade, investigations using genetically-engineered mice have led to new insights into the genetic control of embryonic vascular development, which has also had a major impact on our understanding of neovascularization in many human diseases including cancer, atherosclerosis and diabetes. Micro-imaging methods such as ultrasound biomicroscopy (UBM) and magnetic resonance micro-imaging (micro-MRI) can play an important role in this research, enabling direct in utero visualization of the developing mouse embryo. To date, there has been relatively little progress in the area of molecular imaging with ultrasound and MRI, especially in the area of vascular development. UBM is a real time imaging method enabling noninvasive in vivo analysis of mouse embryonic cardiovascular anatomy and hemodynamics, and can also be applied for image-guided intravascular injection of contrast agents. Micro-MRI provides better 3D resolution and more flexibility than UBM in manipulating cellular/tissue contrast, including more available contrast agents and approaches for cell-targeted imaging, but requires longer acquisition times, and has only recently been demonstrated for effective in utero imaging of mouse embryos. Several reports have recently demonstrated that biotinylation of cell surfaces can be achieved, allowing cell-targeted imaging with avidin-conjugated contrast agents, which are now available for both ultrasound and MRI. This is an attractive option for imaging vascular endothelial cells (VECs), since contrast agents can be delivered to the cells of interest via intravascular injection, even at embryonic stages of development. Moreover, the binding between avidin and biotin is the strongest found in nature, which should make it possible to label vascular cells even in the face of high wall shear rates associated with arterial blood flow. The specific aims of this project are: 1) To optimize the micro-MRI protocols required for in utero analysis of cardiovascular development; 2) To produce transgenic mice designed for targeted imaging of VECs with UBM and micro-MRI; and 3) To establish VEC-targeted micro-MRI approaches for improved analyses of embryonic vasculature. The approaches developed in this project will provide powerful new tools for direct analysis of vascular development in living mouse embryos. Significantly, these new molecular imaging methods will provide, for the first time, the ability to detect vascular gene expression in utero in normal and genetically-engineered mice.